Abstract/Details

HYDROGEN ABSORPTION PROPERTIES AND MAGNETIC BEHAVIOR OF RARE EARTH - TRANSITION METAL BORIDES

SPADA, FREDERICK EDSON.   University of California, San Diego ProQuest Dissertation & Theses,  1983. 0552128.

Abstract (summary)

The hydrogen absorption properties of the hexagonal CeCo(,4)B-type ternary borides R Co(,4)B (R = La, Pr, Sm) and RNi(,4)B (R= Ca, La, Pr), as well as the related ternary La(,3)Ni(,13)B(,2) (LaNi(,4.33)B(,0.67)), have been investigated. Maximum hydrogen uptakes are given by RCo(,4)BH(,4.5), CaNi(,4)BH(,3.2), RNi(,4)BH(,1.5) (R = La, Pr), and La(,3)Ni(,13)B(,2)H(,13.7)(LaNi(,4.33)B(,0.67)H(,4.6)). Phenomenological chemical rules are used to describe the observed hydrogen capacities. These rules suggest that the R-B planes do not accommodate hydrogen. In the RCo(,4)B-H(,2) system, three distinct hydride phases exist at ambient temperature in the pressure range 0-100 atm. Models for hydrogen site occupancy in these phases are proposed and tested via configurational entropy calculations.

Emphasis is given to the hydride LaNi(,4)BH(,1.5) which is used as a model to further understand hydrogen diffusion in CaCu(,5)-type materials such as LaNi(,5-x)Al(,x)H(,z). NMR measurements of the proton rigid-lattice second moment confirm that hydrogen does not occupy sites in the vicinity of the La-B planes. These planes consequently act as barriers to hydrogen diffusion parallel to the {001} direction and limit hydrogen mobility. Reduced hydrogen diffusion is not responsible for the slow LaNi(,4)B-H(,2) reaction kinetics, however; measurements suggest that the rate-limiting step is a surface process.

In related studies, a single crystal investigation of the LaNI(,4)B structure indicates that the CeCo(,4)B atomic arrangement is only a subcell in a larger superstructure extending along the {100} direction. The expressions a(,0) = 6a(,0)('') and c(,0) = c(,0)('') describe the relationship between the lattice parameters of the CeCo(,4)B-type subcell and those of the superstructure.

Magnetic behavior of the CeCo(,4)B-type materials R(M(,1-x)M(,x)(''))(,4)B with R = rare earth and M,M' = Fe, Co, Ni is also described. Ternary and pseudoternary materials with R = Sm exhibit giant intrinsic magnetic hardness. Coercivity in SmCo(,4)B is dominated by nucleation, whereas pseudoternaries show hardness due to pinning mechanisms. Pseudoternaries containing Fe exhibit properties amendable to permanent magnet applications.

Indexing (details)


Subject
Chemistry;
Physical chemistry
Classification
0494: Physical chemistry
0485: Chemistry
Identifier / keyword
Pure sciences
Title
HYDROGEN ABSORPTION PROPERTIES AND MAGNETIC BEHAVIOR OF RARE EARTH - TRANSITION METAL BORIDES
Author
SPADA, FREDERICK EDSON
Number of pages
1
Degree date
1983
School code
0033
Source
DAI-B 44/04, Dissertation Abstracts International
ISBN
9798660554179
University/institution
University of California, San Diego
University location
United States -- California
Degree
Ph.D.
Source type
Dissertation or Thesis
Language
English
Document type
Dissertation/Thesis
Dissertation/thesis number
0552128
ProQuest document ID
303124746
Copyright
Database copyright ProQuest LLC; ProQuest does not claim copyright in the individual underlying works.
Document URL
https://www.proquest.com/docview/303124746/abstract